The invention relates generally to laser altimeters and in particular to space-borne laser altimeters for mapping planetary surfaces and the like.
Spaceborne laser altimeters typically use modest energy 50 to 100 milliJoules solid state lasers, large telescopes having apertures of 50 to 100 centimeters in diameter, and high detection thresholds to achieve unambiguous surface returns with few or no false alarm resulting from solar background noise. As a result of this conventional design philosophy, spacecraft prime power and weight constrains typically restrict operations to modest repetition rates on the order of a few tens of Hz which, at a typical earth orbit round velocity of seven kilometers per second limits along-track spatial sampling to one sample every few hundred meters. There is great motivation in obtaining higher along-track resolution and/or better cross-track coverage, but achieving this capability through a simple scaling of the laser fire rate or power is not practical from spacecraft. This is especially true of altimeters for use in orbit about other planets where instrument mass and prime power usage is severely restricted. In ground-based systems the conventional high signals-to-noise ratio approach to laser altimetry does not make efficient use of the available laser photons.
The surface return rate of an Earth orbiting altimeter can be increased by the two orders of magnitude for a given laser output power by emitting the available photons in a high frequency (few kilohertz) train of low energy, approximately one milliJoules pulses as opposed to a low frequency train of high energy pulses by employing single photon detection. This mode of operations reduces the risk of internal optical damage to the laser, thereby improving long-term reliability and makes the beam inherently more eyesafe to a ground-based observer. In addition these high return rates can be accomplished with much smaller telescope apertures on the order of 10 centimeters diameter. Indeed the contrast of the terrain signal against the solar-induced noise background is actually enhanced through the use of a small receive telescope.
However a problem with such a ground-based system is that it relies on the accuracy smoothness or reliability of a satellite track. Such a system is unsuitable for use in an orbiting altimeter for providing high resolution of rapid terrain changes across steep slopes. The ground-based system in part obtains its relatively large signal-to-noise ratio by means of the predictability of the orbital motion or velocity of displacement from the satellite to the receiving head along the path of the laser beam.
What is needed then is an improved microaltimeter system for use in spacecraft and in particular in deep space probes. The system should use relatively low power but which provide very high resolution of terrain, atmospheric, oceanic features and the like.
Apparatus embodying the present invention includes a microlaser associated with a small diameter telescope. The microlaser emits pulses of coherent light at a high repetition rate. The emitted laser pulses are collimated by the telescope and transmitted to a ground track. Photons are reflected off the ground. The return photons are filtered through a spectral filter and through a spacial filter to reduce solar background noise. The telescope then receives the return photons and supplies them to a multi-pixel or a multi-faceted single photon detection system.
The use of relatively simple software algorithons based on post-detection Poisson filtering techniques enables the identification and extraction of surface sampling data from the more abundant optical background noise prior to on-board storage or transmission of a data to the ground station.
The roughly order of magnitude reduction in required telescope diameter greatly simplifies the mechanics of scanned system and allows the use of relatively inexpensive, modest diameter optical wedges or holographic optical elements to simultaneously scan boet the transit and receive beams for-cross-track interrogation of the terrain while maintaining narrow fields of view for background noise suppression.
By using state of the art photon counting detectors, which are capable of providing centimeter level ranging resolution, as well as angularly resolving the source of a single photon event within the receiver field-of-view performance is further enhanced. With high angular resolution of the single photon source the measured range becomes nearly a point-to-point measurement. That is from an internal altimeter reference point to a small area of uncertainty on the surface. This is determined by the angular resolving power of the photodetector and can be very small compared to the total beam area on the ground. The ranging precision then is limited by the laser pulsewidth, the timing capabilities of the range receiver, and the much-reduced residual spreading caused by the roughness of the surface and slope within the very small zone of range uncertainty. This ability to measure the near point-to-point-time-of-light of an individual photon, together with the high effective signal to noise the ratio, avoids much of the range ambiguity inherent in current high energy altimeters. Such systems will require multiple photons reflected from anywhere within illuminated spot to be recorded by waveform digitizers which consume relatively large amounts of power. The signals would then have to be deconvoluted using sophisticated and not completely reliable algorithms in order to decipher the results and obtain a single range measurement.
The apparatus and method embodying the present invention will provide significantly greater spatial resolution in either the along-track or cross-track directions or both, as well as greatly reduced demands on spacecraft resources such as prime power, volume and weight allocations. Potential altimeter targets are land, ice, and water surfaces as well as distributed or soft targets such as clouds, planetary boundary layers, tree canopies and other vegetation. The much-reduced signal levels of the microaltimeter embodying the present invention relative to conventional altimeters are largely offset by a corresponding reduction in the detection thresholds to one photoelectron or less so that instrument sensitivity is enhanced. As a result geoscience applications including development of high resolution, high accuracy topographic databases of land surfaces useful for studying hydrogical runoff, the effects of clouds on radiation balance, changes in sea, lake, or reservoir levels, changes in ice sheet thickness, tree canopy heights and biomass assessment are all possible through use of the apparatus and method embodying the present invention. Applications to extraterrestrial science missions including low power high resolution topographic mapping of other planets, moon, asteroids and comets within the Solar Systems may also be carried out through he apparatus and method of this invention. The apparatus and method of this invention may also be used for aerial surveying of cities and towns and/or generating of local topographic maps from high aircraft cruise altitudes which do not require special Federal Aviation Administration waiver.
It is a principal aspect of the present invention to provide an apparatus and method for low power, highly accurate ranging detection from a high altitude aircraft or an orbiting spacecraft or the like.
Other aspects of the invention will become obvious to one of ordinary skill in the art upon a perusal of the following specification and claims in light of the accompanying and drawings.